The present invention generally relates to communications systems and, more particularly, to wireless systems, e.g., terrestrial broadcast, cellular, Wireless-Fidelity (Wi-Fi), satellite, etc.
Digital Video Broadcasting-Terrestrial (DVB-T) (e.g., see ETSI EN 300 744 V1.4.1 (2001-01), Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television), is one of the four kinds of digital television (DTV) broadcasting standards in the world, and DVB-H is a standard for handheld applications based on DVB-T (also referred to herein as DVB-T/H). DVB-T uses Orthogonal Frequency Division Multiplexing (OFDM) technology, i.e., DVB-T uses a form of a multi-carrier transmission comprising many low symbol rate subcarriers that are orthogonal.
OFDM technology provides high data rate wireless communications. In an OFDM-based communication system, it is crucial for the receiver to determine channel state information for every subcarrier. Channel state information represents the degree of confidence in each subcarrier for reliably transmitting data.
A conventional channel estimation arrangement is shown in FIGS. 1 and 2. In DVB-T there are two modes of operation, a 2K mode—corresponding to the use of 2048 subcarriers—and an 8K mode—corresponding to the use of 8192 subcarriers. In this example, it is assumed that the receiver is operating in the 8K mode. Operation in the 2K mode is similar and not described herein. The channel estimation arrangement of FIG. 1 comprises Fast Fourier Transform (FFT) element 105, carrier phase error (CPE) removal element 110 and channel estimation and equalization (CHE) element 115. FFT element 105 processes a received baseband signal 104. The latter is provided by, e.g., a tuner (not shown) tuned to a selected RF channel. FFT element 105 transforms received baseband signal 104 from the time domain to the frequency domain and provides an FFT output signal 106. It should be noted that FFT output signal 106 represents complex signals having in-phase and quadrature components. Typically, FFT element 105 performs butterfly calculations as known in the art and provides reordered output data (8192 complex samples in an 8 k mode of operation). As such, FFT element 105 may additionally perform spectrum shifting to rearrange, or shift, the FFT output data to comply with subcarrier locations in accordance with the above-mentioned DVB-T standard. CPE removal element 110 processes FFT output signal 106 to remove any carrier phase error and provides a CPE corrected signal 111 to CHE element 115. CHE element 115 processes the CPE corrected signal 111 for (a) determining channel state information (CSI) for providing CSI signal 117; and (b) equalizing the received baseband signal to compensate for any transmission channel distortion for providing equalized signal 116. As known in the art, CSI signal 117 may be used for obtaining bit metrics for use in decoding (not shown in FIG. 1). Equalized signal 116 is further processed by the receiver to, e.g., recover content conveyed therein (audio, video, etc.) (also not shown in FIG. 1).
Turning now to FIG. 2, the operation of CHE element 115 is shown in more detail. CHE element 115 comprises pre-process element 150, time interpolator 155, frequency interpolator 160, data buffer 165 and equalizer 170. Data buffer 165 simply delays CPE corrected signal 111 before processing by equalizer 170 while the CSI information is determined by the elements in the lower processing path (pre-process element 150, time interpolator 155 and frequency interpolator 160). As noted above, equalizer 170 equalizes the received baseband signal (e.g., the delayed version of CPE corrected signal 111) to compensate for any transmission channel distortion for providing equalized signal 116.
In terms of the lower processing path, the channel estimation process utilizes the pilot signals present in DVB-T. In particular, in DVB-T there are two types of pilots: scattered pilots (SP) and continual pilots (CP), and the channel estimation process uses interpolation to estimate the channel state information (CSI) of the subcarriers from the SPs. First, pre-process element 150 processes CPE corrected signal 111 to determine the CSI of the received SPs. Since the pilots are transmitted with known values, pre-process element 150 processes the received SPs relative to their known values to determine their channel state information, which is provided via pre-process output signal 151. The CSI of the SPs (151) are then processed by time interpolator 155. In particular; time interpolator 155 interpolates (in the time domain) the CSI of every third subcarrier and provides output signal 156 (which includes the CSI of the SPs and the newly time interpolated CSI of every third subcarrier). Finally, frequency interpolator 160 processes output signal 156. In particular, frequency interpolator 160 interpolates (in the frequency domain) the CSI of all of the subcarriers (in effect smoothing the previously determined CSI of, e.g., the SPs) and provides CSI signal 117 (which provides the CSI for all subcarriers). Equalizer 170 utilizes CSI signal 117 to perform the above-described equalization of the received baseband signal and, as also noted above, CSI signal 117 may be used for obtaining bit metrics for use in decoding.